Energy absorbing seat belt retractor having a torsion bar

ABSTRACT

A torsion bar ( 50 ) and seat belt retractor ( 20 ). The torsion bar ( 50 ) has a circular cross section where the torsion bar is formed as a cold headed extruded part and wherein the grain structure of the metal is aligned along the axis (axial direction) of the bar generally in the center of the bar and the grain structure smoothly diverges from the axial direction near an end formation ( 52   a,b ). The torsion bar is later annealed and twisted about 180 degrees. 
     The seat belt retractor ( 20 ) includes: a frame ( 22 ) to rotationally support the torsion bar and spool ( 30 ). The torsion bar ( 30 ) is characterized by an elastic deformation zone and a sharp onset into a plastic deformation zone. The spool is operatively connected to rotate with the torsion bar. The retractor also includes a locking device, activated during a vehicle accident and operatively linked to the torsion bar for preventing one side of the torsion bar from rotating while permitting the other side and the spool to rotate once loaded by the occupant.

This application is a division of application serial no. U.S. Ser. No. 09/170,409, filed Oct. 13, 1998, now U.S. Pat. No. 6,065,706 which is a continuation of application serial no. U.S. Ser. No. 09/060,241, filed Apr. 14, 1998, now U.S. Pat. No. 5,899,402.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention generally relates to torsion bars usable in energy absorbing seat belt retractors.

The classic type of seat belt retractor comprises a frame with a spool rotationally mounted upon the frame. The spool will typically include one or more lock wheels each having a plurality of teeth which are engaged and locked by a corresponding lock pawl. The lock pawl or lock dog is rotationally mounted to the frame and movable from a disengaged position to an engaged position with a tooth of the lock wheel. In this type of retractor once the spool is locked, further rotation of the spool is prohibited. One skilled in the art will appreciate that all forward motion of the occupant will not be stopped in this type of retractor because as the occupant loads the locked retractor, the seat belt is stressed and stretches and the seat belt slips over itself (the so called film spool effect).

However, with an energy absorbing retractor, the spool and its associated mechanisms are permitted to rotate and the seat belt is controllably permitted to protract in response to the load imparted to the seat belt by the occupant. The forward motion of the occupant is restricted by a reaction force or torque generated within the retractor and modified by the stretching seat belt. In this way the protraction of the seat belt and the forward motion of the occupant are controlled. Energy absorbing seat belt retractors often employ a deformable member such as a crushable bushing or a torsion bar. In either case, the bushing is crushed or the torsion bar twisted beyond its elastic limit into its plastic range or zone of operation to generate the desired (theoretically constant) reaction torque which acts against the torque transferred to the retractor spool via the forces imparted to the seat belt by the moving occupant.

The goal of an energy absorbing retractor is to generate a generally constant reaction force to oppose the forward motion of the occupant and to be able to generate this reaction force during the accident event, that is, during the entire time that the seat belt is loaded by the occupant. In theory this can be achieved by utilizing a crush bushing or torsion bar that always operates in its constant plastic zone.

In a torsion bar, seat belt retractor, one end of the torsion bar is fixedly attached to a lock wheel and the other end is fixed to the retractor spool. During an accident the lock wheel is prevented from rotating by interposing a lock dog or lock pawl within the teeth of the lock wheel. As the seat belt is loaded by the occupant, the spool will tend to rotate in opposition to the reaction torque generated within the torsion bar, as the torsion bar is twisted. The generated reaction torque depends upon the amount that the torsion bar is rotated or twisted as well as upon the physical characteristics of the torsion bar.

More specifically, the reaction torque generated by a torsion bar will vary depending upon whether the torsion bar is in its elastic, transition or plastic zones or ranges. As mentioned, in an ideal torsion bar, the elastic range is characterized by a steep (preferably infinitely steep slope or deflection curve) and the plastic range is characterized by a perfectly constant torque deflection region having a sharp transition from the elastic region. In this ideal torsion bar and corresponding seat belt retractor, once a first end of the torsion bar is locked and the spool loaded, the torsion bar will immediately make a transition from its elastic range (see curve 100 of FIG. 1) into the plastic range of operation such that a constant reaction force is generated by the retractor as the seat belt is protracted.

Prior art torsion bars have been made using a number of different manufacturing methods. In one method, an over-sized metal bar is machined to reduce its diameter to a desired dimension. Subsequently, end formations are formed on the machined bar such as by cold rolling. The machining of the bar may produce stress risers which are typically non-uniform and the cold rolling of the machined bar, it is believed, reorients the grain structure of the metal in an undesirable manner. To make the stress distribution within the torsion bar more uniform, an annealing step is often used, which adds to the cost of the final product. However, this type of torsion bar does not achieve the objects of the present invention as it displays the characteristic torque deflection curve similar to that shown in curve 102 of FIG. 1 having an elastic zone, an extended elastic/plastic transition zone and a plastic zone. In another method of manufacture the torsion bar is made using a cold-formed process in which a metal bar or wire (large diameter), has a diameter less than the desired dimension. The smaller than desired diameter bar is expanded into a bar having the desired larger diameter. This type of bar has been tested and it displays or shows a characteristic torque deflection curve similar to that of curve 102 of FIG. 1. The prior art has also suggested a method of making a torsion bar having a shortened or abrupt elastic/plastic transition zone. In this method a pre-machined or pre-formed torsion bar is work hardened (by being pre-torqued or twisted beyond its yield torque level) prior to installation within a seat belt retractor. One potential deficiency of this technique is that the pre-twisting reduces the useful range through which the torsion bar can be additionally twisted, during an accident, once installed within a retractor.

I have previously proposed another methodology of making a torsion bar for use within a seat belt retractor. The torsion bar was formed of a ductile, elongated body, located between the end formations and formed by pre-stressing bar stock by extruding an oversized metal bar into a bar of a reduced diameter with its grain structure in the vicinity of a center of the bar aligned in a longitudinal direction. The end formations of the torsion bar were formed by a cold heading process. In this process the cold headed bar was not annealed. In this process, the cold heading did not disturb the longitudinal direction of the grain structure in the central portion of the torsion bar.

I have found that if the bar stock is first extruded, with the formations cold headed as I had first proposed but subsequently if the torsion bar is annealed but not at a temperature or duration which will not cause the grain structure to increase and the torsion bar is pre-twisted, excellent results can be expected.

It is an object of the present invention to provide a torsion bar which displays an abrupt transition from its elastic zone to its plastic zone. A further object of the invention is to provide an energy absorbing seat belt retractor which uses the above type of torsion bar.

Accordingly, the invention comprises: a torsion bar having a circular cross section where the torsion bar is formed as a cold headed extruded part. Preferably, the grain structure of the metal is aligned along the axis (axial direction) of the bar. If the grain structure diverges from the axial direction near an end formation it does so smoothly. Subsequent to being cold headed, the bar is annealed and thereafter twisted to generate a determinable level of work hardening. For the disclosed bar the amount of twist is about 180 degrees (0.5 twist).

Additionally, the invention comprises a seat belt retractor having: a frame; wherein the torsion bar is rotationally supported relative to the frame and wherein the torsion bar is capable of generating a determinable reaction torque as it is twisted (with one end thereof fixed). The torsion bar is characterized by an elastic deformation zone and a sharp onset into a plastic deformation zone. The retractor also includes a spool operatively connected to rotate with the torsion bar; lock means, adaptable during a vehicle accident and operatively linked to the torsion bar for preventing one side of the torsion bar from rotating while permitting the other side and the spool to rotate once loaded by the occupant.

Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a torque-deflection curve for an idealized torsion bar and for a conventional torsion bar having a circular cross section.

FIG. 2 shows test data illustrating a torque-deflection curve for a torsion bar that has been cold-formed and annealed.

FIG. 2a shows test data for a torsion bar that has been cold formed, annealed and pre-twisted.

FIGS. 3a and 3 b show a torsion bar in various steps of completion.

FIGS. 4a and 4 b are end plan views of the torsion bar.

FIG. 5 shows a seat belt retractor incorporating a torsion bar of the present invention.

FIG. 6 shows a curve of belt force versus time.

DETAILED DESCRIPTION OF THE DRAWINGS

Reference is now made to FIGS. 3a and 3 b which show a torsion bar made in accordance with the present invention. The torsion bar 50 includes a center body 52 and two end formations 52 a and 52 b which enable the torsion bar to be respectively mated with a spring arbor and a ratchet body. The torsion bar 50 is made from a bar (or large diameter wire) of metal having a circular diameter. In the preferred embodiment the bar or wire is made from AIAI/SAE 1005 Modified (CHQ) grade wire or stock. The wire or stock is formed using a known hot rolled, aluminum killed fine grain spheridized annealed steel with a Rockwell B rating of between 50 and 70 and preferably in the range of 60-65. This metal should have a surface finish of 0.1 mm (0.004 inch) maximum seam depth. The tensile strength should be between 40000 and 60000 pounds per square inch. The maximum chemical constituents of the bar stock are preferably: Carbon 0.06, MN 0.35, Phosphorous 0.02, Sulfur 0.02, SI 0.1, CU 0.01, CR 0.08, MO 0.04 and AL 0.06. The elongation is 25% at 2 inches. The maximum grain size is 5. The size of the bar stock is between 9.525 and 11.099 mm (0.375 and 0.437 inches). This metal is chosen because it is sufficiently ductile with its grain structure generally longitudinally aligned.

The initial diameter D_(i) (see FIG. 3a) of the metal bar 110 is approximately five percent (5%) greater than the desired or final diameter D_(d) of the torsion bar 50. The final diameter of the torsion bar is about 1 cm (9.8 mm). The final diameter of the torsion bar is achieved using a cold forming extrusion process in which the bar 110 (see FIG. 3a) is partially forced through a die (not shown) and then bar 110 is cut to the desired length. During this extrusion process or step, the grain structure of the metal bar 110 remains oriented in a generally longitudinal direction. Numeral 53 diagrammatically illustrates a typical pattern of a longitudinal grain structure. That is, the grain structure lies parallel to the axis 112 of the torsion bar 50. Even if the grain structure of the original bar or wire stock does not have its grain structure longitudinally aligned this extrusion process will so align the grain structure.

Subsequent to reducing the diameter of the bar 110 to the desired dimension, the ends of the bar 110 are cold headed to form the end formations 52 a and 52 b. These formations can be formed in a multi-step process or in a single step. FIGS. 4a and 4 b show end views of each of the end formations. The end formation 52 a includes an enlarged diameter section upon which is formed a plurality of splines 56 a. The end formation 52 b includes an enlarged section having splines 56 and at least one integrally formed notch or groove 58 which provides a means to receive a spring arbor 60 as shown in FIG. 5. The cold headed process used to form the enlarged diameter splined sections 56 a and 56 b will maintain the longitudinal grain structure in the bar as well as add a radial component to the grain structure of the material. However, it is believed the transition radii 55 and 57 (see FIG. 3b) prevent any discontinuities in the grain structure and stress distribution. Subsequently, the above-formed torsion bar is annealed at a temperature and duration which does not disturb the grain size of the bar. Unfortunately, the annealed torsion bar does not provide the precise torque-deflection curve that is desired. Characteristically, this curve (see the test data of FIG. 2) exhibits a low yield point. The real-world consequence of this low yield point is that the annealed torsion bar may yield prematurely.

Subsequent to annealing the torsion bar is again pre-stressed by twisting. For the above described bar material (formed into a circular bar of 9.8 mm diameter torsion bar) and overall length of 58.3 mm (from tip-to-tip of the end formations) the bar is twisted 0.5 revolution. The resulting test data is shown in FIG. 2a. As can be seen the low yield point exhibited in FIG. 2 has been eliminated. The torque generated by the bar in its plastic regions increases somewhat linearly at about a slope of 0.0067 Nm per degree. Subsequent testing of this torsion bar has confirmed that the effect of removing the low yield point improves the kinematic performance in a crash. Reference is again made to the test data of FIG. 2a which shows a dramatically reduced transition zone between elastic and plastic behavior of the torsion bar 70 and which has been achieved with pre-twisting as described above. The amount of pre-twist will most probably vary with the diameter, length and material choice of the bar.

Reference is made to FIG. 5 which generally shows the construction of the major components of a torsion bar, energy absorbing seat belt retractor 20. The retractor 20 comprises a frame 22 with first and second sides 24 a, and 24 b and a back 24 c, each of the first and second sides includes a respective first opening 28 a or 28 b. The retractor 20 also includes a hollow spool 30 rotationally supported upon the frame 22. The spool 30 includes a center body 32 and opposing flanges 34 a and 34 b at respective ends of the center body 32. The center body 32 includes a hollow bore 40 having splines 42 formed at one end thereof. The body 32 also includes means such as a slot (not shown) of known construction for receiving and securing an end of a length of seat belt (seat belt webbing) 36. Numeral 36 a designates a few layers of the seat belt 36 wound about the spool.

A torsion bar 50 is received within the bore 40. The torsion bar includes a center body 52 and the end formations 52 a and 52 b. As mentioned, end formation 52 b includes splines 56 (which drivingly engage with splines 42 of the spool). The notch or groove 58 (also see FIG. 4b) receives a driving key 60 of a spring arbor 62. The spring arbor 62 includes a slot 64 in which is received an inner end of a rewind spring 66. The outer end of the spring 66 is fixedly secured to a spring cover 68. The cover 68 is secured to frame side 24 b and includes a circular projection 70 received within frame opening 28 b. The projection 70 serves as a bushing to rotationally support the spring arbor 62, torsion bar 50 and spool 30. The spool 30 includes opposing pockets 38 located adjacent the splines 42 of the torsion bar 50. With the torsion bar in place, a tool is inserted into the pockets to locally deform the spool 30 to crimp the spool splines 42 and the torsion bar splines 56 b together.

Emergency locking retractors (ELRs) include a variety of ratchet or lock wheel assemblies. The precise type for use in the present invention is not particularly important. As is known in the art, the ratchet wheel assemblies include a sensor means for causing a locking pawl to be brought into engagement with teeth on the ratchet or lock wheel to halt the protraction of the seat belt. Such means typically include the use of a vehicle or inertia sensor to sense vehicle deceleration above a predetermined level and a web sensor which is activated to initiate the locking of the retractor when the seat belt (webbing) is withdrawn from the spool at a rate in excess of a determinable level. The ratchet or lock wheel assemblies may use one or more plastic sensor pawls which engage a plastic or metal ratchet wheel which in turn couples a lock cup to the retractor shaft (in the present case to the torsion bar). Having coupled the lock cup to the shaft (torsion bar) the lock cup rotates. The motion of the lock cup moves a load absorbing, typically metal, locking pawl into engagement with a load absorbing metal lock wheel, thus halting, if only temporarily (when using energy absorbing components such as a torsion bar), the protraction of the seat belt. One such lock wheel assembly that is usable with the present invention is disclosed in U.S. Pat. No. 5,529,258 or EP 0228729 which are incorporated herein by reference.

The end formation 52 a of the torsion bar 50 is secured to a ratchet wheel assembly 80. The assembly 80 includes a ratchet body 82 and lock or ratchet wheel 84 having teeth 85. The ratchet body includes a tubular portion 86 having internal splines 88 which engage the splines 56 a of the torsion bar 50. The lock wheel 88 may be a part of the body 82 or a separate part that is staked thereto as illustrated. The ratchet body is received within frame opening 28 a and is supported by a bushing 90. A locking pawl 92 is rotationally supported upon the frame side 24 a and is movable into engagement with the teeth 85 of the lock wheel 84 in response to the activation of a vehicle or web sensor.

The lock wheel assembly 80 includes a web sensor 220 that is coupled to sense the angular acceleration of rotation of the spool 30. As illustrated, the web sensor is coupled to the torsion bar 50 via the lock wheel assembly 80, the speed of which (prior to lockup) is that of the spool. The lock wheel assembly further includes a vehicle sensor 222. As mentioned above, the specific implementation of the web and vehicle sensors will vary, however, this is known in the art. Whenever either the vehicle or the web sensor is activated the lock pawl 92 is brought, via known mechanisms, into locking engagement with a lock wheel 84.

The operation of the retractor 20 is generally the same of that outlined above. During an accident, the end 52 a of the torsion bar 50 is locked from further rotation and the seat belt is loaded as the occupant moves or attempts to move forward. The occupant load is transferred to the spool 30, via the belt 36, whose motion is opposed by the reaction torque generated as the spring end 52 b of the torsion bar is rotated. Increased occupant load will cause the spool 30 and the torsion bar to rotate, in opposition to the reaction force, thereby protracting the seat belt 36 and permitting the occupant to move forward in a controlled manner.

Various inventors have referred to energy absorbing seat belt retractors as constant force retractors. This reference is presumably to a theoretical constant plastic reaction torque (or force) that is achieved when the energy absorbing device, such as the torsion bar or crush ring, is deformed into its plastic region. Having generated this constant torque at for example the torsion bar, this force is communicated to the retractor spool and then to the seat belt. However, if the goal is to produce a retractor that displays a generally constant reaction force, even using a perfect torsion bar will not permit this. This can be seen by the following. The reaction force, F, on the seat belt is equal, in the steady state, to F=0.5*D*T where F is the reaction force measured at the belt, D is the effective diameter of the spool plus any roll of seat belt webbing thereon and F is the reaction torque generated by the torsion bar. As the occupant loads the spool, the torsion bar begins to twist and generate a reaction torque. However, as the spool twists, more seat belt webbing is protracted off from the spool and the effective diameter, D, reduces. Consequently, even if the torque, T, is constant the belt reaction force may vary in correspondence to the belt removed from the spool.

FIG. 6 is test data for a crash simulation of using a 95^(th) percentile hybrid III dummy and the above described torsion bar retractor. This test data shows that a retractor using the present invention can generate a reaction force that is remarkably constant. Prior to this test the seat belt webbing was extracted from the retractor and secured about the dummy so that about 3-4 layers of seat belt remained rolled on the spool. The effective diameter prior to testing was D=D_(s)+D_(w), where D_(s) is the fixed diameter of the spool which is 0.41 mm and D_(w) was the added width dimension due to the remaining 3-4 layers of seat belt on the spool. In this test condition D=50 mm. The seat belt used was conventional woven polyester seat belt material with an elongation of about 6% and a thickness of about 1.27 mm. The resulting combination provided the near constant reaction force.

Many changes and modifications in the above-described embodiment of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, that scope is intended to be limited only by the scope of the appended claims. 

What is claimed is:
 1. An energy absorbing seat belt retractor (20) comprising: a spool (30) and a torsion bar (50), the torsion bar including first and second end formations (52 b and 52 a), the first one of the end formations (52 b) is drivingly connected to the spool; the torsion bar further includes a ductile, elongated body (52), located between the end formations, the bar having its grain structure in the vicinity of a center of the bar (50) aligned in a longitudinal direction.
 2. The device as defined in claim 1, wherein the end formations (52 a and 52) are formed by a cold heading process.
 3. The device as defined in claim 2, wherein the second end formation is connected to a lock wheel assembly means (84) for locking during an accident to prohibit the second end of the torsion bar from rotating while permitting the first end and the spool to rotate while permitting a seat belt (36) wound about the spool to be controllably protracted.
 4. The device as defined in claim 1 wherein the torsion bar means is subjected to a pre-stress, prior to installation within the retractor sufficient to align the grain structure of the bar longitudinally.
 5. The retractor as defined in claim 1 wherein the torsion bar has a circular cross section.
 6. A method of forming a torsion bar (50) comprising the steps of: a) aligning the grain structure on a metal bar, of predetermined diameter, in an axial direction; b) forming on one end of the bar a first driving formation, c) forming on the other end of the bar a second driving formation.
 7. The method of claim 1 wherein the steps of forming the first and second driving formation includes forming by a cold heading process. 